Paper, tissue, board and other so-called “web” products may be produced in a web production system controlled by a process control system. The appearance of web products produced by the web production system may be measured in a laboratory after a production run by removing one or more samples of the produced product from a machine reel holding the product. A machine reel may also be referred to as a take-up reel, or simply as a reel. The appearance of the web product may be described by measurable characteristics that include opacity, brightness, color, fluorescent properties, whiteness, gloss, and uniformity, as well as other characteristics. As used in this disclosure, the term “appearance” indicates color and/or other optically observable characteristic(s) of web products.
Typically, laboratory color measurements are made by stacking a predetermined number of sample pieces of the product and placing the stack in a measurement device. The measurement device illuminates the sample using a specified geometry with one or more light sources having specified characteristics, with or without specified filters. Light reflected and emitted from the sample product is measured under specified conditions by a sensor. A color sensor is typically calibrated using prepared calibration tiles of known properties. Such measurement parameters are typically specified by a standards-setting body, such as Commission Internationale de l'Eclairage (CIE) or International Organization for Standardization (ISO), for example ISO 2469 and ISO 5631.
Similarly, under the same or other illumination conditions, other aspects of appearance of the web product sample may be measured. Such appearance characteristics include D65 brightness, CIE whiteness, true reflectance, and apparent reflectance. Appearance of the produced product may be measured under sets of different illumination and viewing conditions, such as UV-included D65, D55, C or UV-excluded or true reflectance.
During production of a web product in a web production system, on-line sensors measure color and other appearance characteristics of the product as it is produced. Because such measurements are made from a moving web, some prerequisites of the standards-specified color measurement conditions and procedures often cannot be met. In one example, only a single sheet of product can be measured, rather than a stack of multiple layers of paper as specified in a standard. In a second example, some specified illumination and diffusion requirements may not be met by on-line appearance sensing measurement systems.
Furthermore, an online measurement system may employ an optical system having different wavelength range or wavelength response characteristics than an optical system employed in the off-line, quality laboratory measurement system. Also, physical properties of the moving web being measured in the web production system may be different than the same properties of the finished product, after conditioning to climate conditions of the quality laboratory. Such properties may include temperature and moisture content of the web product being produced.